The present invention relates to a method for applying a weld overlay to a wastage susceptible structure.
According to U.S. Pat. No. 5,356,248 to Hillestad, steam generating boilers, such as those used at electrical power generating plants, are generally large steel or masonry structures containing hundreds of vertically arranged steel tubes which are in thermal contact with a burning fuel, such as coal. The burning fuel heats water circulated through the tubes. The heated water or steam is used to drive turbines which generate electricity.
Additionally, according to the '248 patent, the tubes are generally organized in large panels or banks of parallel tubes which are connected together with a metal membrane or web continuously interposed between each pair of adjacent tubes in the bank to form a tube wall. In a steam generator, the tubes are generally oriented in a vertical direction and are communicated with headers at their ends. Due to their exposure to relatively high temperature water and steam, these tubes are susceptible to corrosion mechanisms that can ultimately lead to significant tube wall thinning or even leakage.
One approach to repairing or preventatively strengthening the tubes of a steam generator involves the deposition of weld metal at locations of the tubes observed or otherwise judged to have a reduced thickness less than a desired tube thickness. However, the build up of weld metal to thereby form a corrosion resistant weld overlay presents installation and structural challenges. For example, since a field applied weld overlay is typically applied during a shutdown of the steam generator, it is desirable to accomplish the set up and operation of the weld equipment as rapidly as possible so as to limit the revenue loss downtime of the steam generator. The speed of the weld deposition, however, must take into account the tube structure impact which the weld operation may have. For example, tube panels may tend to deform due to the difference in temperature between the furnace side of the tube being welded and the relatively cooler, insulated exterior side of the tube. Moreover, shrinkage of the weld overlay can sometimes engender a deformation of the tube panel, thereby causing increases in the stresses exerted on the structural framework supporting the tubes. Furthermore, residual stress from the welding operation can sometimes result in thermal fatigue cracking of the weld overlay and the tube portions underlying and adjacent the weld overlay region.